1. Field of the Invention
The present invention relates to a circuit substrate containing electrical elements such as capacitance, resistance and inductance elements, and more particularly, it relates to a circuit substrate which comprises a ceramic multilayer structure containing the aforementioned electrical elements formed by discrete chip-like electronic components. More specifically, the present invention relates to an improvement in conductive material for achieving electrical connection with respect to the chip-like electronic components.
2. Description of the Prior Art
In order to implement an electronic circuit with higher density and/or multiple functions, it is advantageous to provide a circuit substrate, on which various electronic components are mounted, with functions for serving as electrical elements such as a capacitor, a resistor and an inductor, in addition to a function of merely holding and interconnecting the electronic components. For example, a substrate consisting of a ceramic multilayer structure, i.e., a ceramic multilayer substrate, is suitable for satisfying the aforementioned requirement.
A report entitled "Multilayer Ceramic Substrate" by Susumu Nishigaki, Electronic Ceramics, Gakken-sha, Vol. 16 (75), May 1985, pp. 61-71 introduces various types of ceramic multilayer substrates. This reports roughly classifies ceramic multilayer substrates into those obtained by the "wet method" employing raw ceramic sheets, i.e., ceramic green sheets; and those obtained by the "dry method" employing fired ceramic plates, on the basis of their manufacturing methods, and further minutely classifies the multilayer substrates obtained by the "wet method" into "green sheet multilayer substrate" and "printed multilayer substrate".
The "green sheet multilayer substrate" is obtained by preparing a plurality of ceramic green sheets printed with thick films of dielectric or insulator paste, resistor paste and/or conductor paste as desired, laminating and pressurizing the green sheets and cofiring the same. Such a multilayer substrate can be formed with circuit elements such as a capacitor, a resistor and an inductor.
The "printed multilayer substrate" is obtained by repeating a process of preparing a green sheet printed with a thick film of resistor paste and/or conductor paste as desired, printing dielectric or insulator paste thereon and drying the same and again printing dielectric or insulator paste, resistor paste and/or conductor paste as desired.
The "dry method" is characterized in that a fired ceramic plate is employed to repeat the step of forming a resistor paste film and/or a conductor paste film thereon by thick film printing as desired, drying and firing the same and the step of forming an insulation layer by a similar method.
However, the aforementioned conventional methods or substrates obtained thereby have the following problems to be solved.
In a ceramic multilayer substrate obtained by the "wet method", including the "green sheet multilayer substrate" and the "printed multilayer substrate", the green sheets, the dielectric or insulator paste film, the resistor paste film and/or the conductor phase film are contracted and deformed, and hence it is difficult to obtain desired design characteristics such as the electrostatic capacitance value, the resistance value and the inductance value of the capacitor element, the resistor element and the inductor element, respectively, formed in the substrate. Particularly the method of manufacturing the "green sheet multilayer substrate" includes a pressurizing step in advance of firing, and the said deformation may also take place in the pressurizing step. Further, the "wet method" for obtaining the multilayer substrate includes a step of firing the ceramic green sheets simultaneously with the dielectric or insulator paste film, the resistor paste film and/or the conductor paste film. Thus, the resistor paste film and/or the conductor paste film as employed are also exposed to high temperature and to the atmosphere in firing. Consequently, the ceramic material for forming the ceramic green sheets or the dielectric paste film must be selected from those which can be fired at such a temperature and in such an atmosphere that the characteristics of the resistor paste and/or the conductor paste do not deteriorate, whereby the range of selection of employable ceramic materials is narrowed. In general, ceramic material which can be fired at a relatively low temperature has a small dielectric constant and hence it is difficult to form a capacitor element having large electrostatic capacitance. To the contrary, the resistor paste film must withstand the aforementioned firing of the ceramic material, and hence it is difficult to select a specific resistance of the resistor forming the resistance element over a wide range.
A specific problem of the "printed multilayer substrate" is that the flatness of the surface to be printed gradually deteriorates as the printing of the dielectric or insulator paste, the resistor paste and/or the conductor paste is repeated. Thus, it is difficult to increase the number of layers of the multilayer substrate. Consequently, it is difficult to render electrodes for forming capacitance in a capacitor element multi-layered, leading to difficulty in formation of a capacitor element having large capacitance in the multilayer substrate. Further, since the surface to be printed gradually loses its flatness, the position and the pattern of printing of the resistor paste and/or the conductor paste may not be obtained according to the desired design. The problem also makes, it difficult to obtain the resistance value, the electrostatic capacitance value and the inductance value according to the desired design.
On the other hand, the "wet method" includes a printing step which is similar to the aforementioned method of obtaining the "printed multilayer substrate", and thereby confronts a problem substantially similar to the aforementioned problem specific to the "printed multilayer substrate".